Allow views attachment display to use its own pager options

Problem/Motivation

1. The views Attachment display has its own options pager configuration, but the pager doesn't display when enabled.
2. Needs to work with ajax, too.

Steps to reproduce

Used devel_generate to create some nodes
See attached yml for basic view config.

1. Add a view (myview) - Content, of Type Article, Create a page, Display format: Unformatted list of titles, Items to display: 5, Use a pager
2. Use a full pager
3. Add Attachment display
4. Attach to: Page; Under Pager section, Items to display: Change to full pager. Items to display: 3, change Pager ID to 1
5. Go to page, note there is no pager for the Attachment.
6. If Inherit pager and Render pager are enabled, then the pager shows up, but it inherits the settings from the Page display instead of using its own pager config.

Other manual testing variations:

• Duplicate view and repeat steps to create other views for testing: 1. enable AJAX on both page and attachment, 2. enable AJAX on just page, and 3. enable AJAX on just attachment.
• Test mini pager
• Test contrib views_infinite_scroll (per comments in #48 and #49, this shouldn't be a show stopper, but it's a common use case, so we should determine if this will break anything in the module.

Proposed resolution

Honor the pager configuration settings for the Attachment display.

Remaining tasks

1. Comprehensive tests, including testing some scenarios that might not be the current use case of the people that run into this
2. Decide: should this ticket address odd behavior when Ajax is enabled on both the attachment and the attached to display, see #55? Or does that belong in its own issue?
3. Is #52 reproducible?